For tribosystems, in particular those such as are used in many technical applications, it is important to use lubricants to reduce the friction and the wear on the contact surfaces of moving parts. In this context, lubricants of different consistency can be used depending on the area of application. Lubricating oils have a liquid and flowable consistency whereas lubricating greases have a semi-solid to solid, frequently gel-like, consistency.
A characteristic feature of a lubricating grease it that a liquid oil component is taken up and retained by a thickener component. The pasty nature of a lubricating grease and its property of being spreadable and plastically easily deformable together with the property of being adhesive ensures that the lubricating grease wets the lubricating point and the lubricating effect develops at the tribologically stressed surfaces.
Lubricating greases generally consist of a thickener which is homogeneously distributed in a base oil. Additional adjuvants such as emulsifiers are frequently used so that the thickener is stably dispersed in the base oil. Various substances are known as base oils. Organic and inorganic compounds are used as thickeners.
The most important rheological properties of a lubricating grease include the consistency or its flow limit, the avoidance of post-curing and excessive oil deposition under thermal and mechanical loading as well as a stable viscosity-temperature behaviour. Frequently at lubricating points sealed against egress of lubricating grease, a thixotropic (shear thinning) and shear-unstable behaviour of the lubricating grease is also advantageous as long as a consistent lubricating grease is only required for the mounting of corresponding components. A high degree of practical experience is required to create a lubricating grease of high usage value depending on the lubricating and equipment requirements.
Lubricating greases are frequently used in encapsulated or sealed environments in order to protect the lubricating point from water, minimise losses of lubricating grease and avoid the ingress of particles such as sand or dust. A typical application is greased joints of constant velocity shafts encapsulated with plastic bellows. In this case, the encapsulating material frequently entrains the movements of the parts which are moving with respect to one another or at least takes up vibrations. For this purpose a mobility and in most cases, also elasticity of the material is required which must not be negatively influenced by the contact or by the interaction with the lubricating grease. It has been observed, however, that conventional lubricating greases attack these encapsulating materials, with the result that these become brittle, for example, and/or are damaged by hydrolytic degradation.
Product solutions and property rights existing for the grease lubrication of constant velocity shafts are predominantly concerned with tribological questions and to a lesser degree with the provision of tribocontacts by thixotropic and/or shear-unstable greases as well as the compatibility of lubricating grease and bellows material. Among other things, the continued miniaturisation of constant velocity shafts in vehicles has led to a demand for an improved lubricant in order to satisfy the higher requirements regarding friction and tribology. The new types of lubricant according to the invention have therefore been developed because they act very well at higher ambient temperatures which are associated with the continuing vehicle development.
For the use of lubricating greases in constant velocity shafts, these must not have too-high flow limits which prevent the lubricating grease from automatically flowing back into the lubricating gap after it has been previously hurled out of the lubricating gap, for example, due to centrifugal forces.
Numerous formulation patents exist which have as their subject matter the combination of lithium, calcium, lithium complex and polyurea thickeners as consistency-giving components.
It is considered to be a shortcoming of this prior art that most lithium or lithium grease complexes and also most lithium/calcium greases have a too-high shear stability with the consequence of increased flexing work and elevated steady-state temperatures in the joint caused by this.
As a result of the greater thermal stressing of the grease and the bellows materials, this brings about a shortened lifetime of the entire system. An inadequate flow of grease causes a deficient supply and therefore inferior wetting of the tribologically stressed surfaces. As a consequence, this leads to increased wear associated with a reduction of the lifetime and a reduced efficiency caused by increased friction.
Commonly used soap thickeners, in particular the lithium and lithium complex greases widely used in joint shafts, as well as polyurea greases have a too-low alkaline buffer effect in order to prevent the formation of free acids from the grease matrix, primarily from the additives, and therefore the acidic hydrolysis of ester-group-containing polymer chains during the ageing of the grease/thermoplastic elastomer (TPE). Furthermore, the excess lithium hydroxide present in the lithium greases in turn ensures alkaline hydrolysis of the ester groups which likewise leads to a loss of the mechanical stability of the bellows. Commonly used urea greases have no alkali reserve or only a very low alkali reserve. Acidic ageing products formed in these greases can only be inadequately neutralised.